1. Field of the Invention
The present invention relates to an immersion nozzle, and more particularly to an immersion nozzle for pouring molten steel into a mold during a continuous casting process, wherein the mold has a mold cavity formed with a substantially rectangular-shaped horizontal cross-section having a long side of 2000 mm or more and a short side of 150 mm or less.
2. Description of the Related Art
Heretofore, in a mold for receiving therein molten steel to produce a steel plate, so-called “slab”, during a continuous casting process, a mold cavity has been generally designed to have a width dimension of less than about 2000 mm. Recently, there has emerged a high-speed casting operation using a mold having a wide mold cavity, i.e., a mold cavity with a larger width dimension, more specifically, a mold cavity formed with a substantially rectangular-shaped horizontal cross-section having a long side (i.e., width) of about 2000 mm or more and a short side (i.e., thickness) of about 150 mm or less.
In an operation of pouring molten steel into such a wide mold cavity, a molten steel flow discharged from an outlet port of an immersion nozzle (i.e., submerged nozzle) will be spread and decelerated in a vicinity of a lateral end of the mold cavity, and further deflected downwardly below the outlet port due to extraction of a slab from the mold. Thus, a stagnant region having poor fluidity is liable to occur in an upper region of the lateral end of the mold cavity. Moreover, a molten steel flow in the mold cavity is apt to become unstable due to episodic occurrence of turbulences therein, such as reversed flows in various regions of the mold cavity and locally deflected flows which frequently change with time, and resulting fluctuation (“wave”, “heave”, “change in flow direction”) in a molten steel surface, to cause difficulty in allowing inclusions around a lateral end of a slab to sufficiently float up and in allowing a mold powder to be uniformly transferred onto a surface of the slab, which leads to uneven incorporation of the mold powder and the inclusions into the slab. The unstable molten steel flow causes another problem about difficulty in obtaining a temperature distribution of molten steel in the mold cavity requited for or optimal to formation of a shell (i.e., primary solidification shell) of a slab during a course of solidification the molten steel. This exerts a negative impact on quality of a slab and increases the risk of break (e.g., cracks) of a slab.
In order to solve the above problems, it is necessary to stably form and maintain a molten steel flow, such as an upward flow in the lateral end of the mold cavity, and a flow directed toward a center of the mold cavity along a vicinity of a molten steel surface in the entire mold cavity, i.e., a reversed flow, while minimizing deceleration of the molten steel flow, even in the lateral end of the mold cavity. From a practical standpoint, even if only dimensions of an immersion nozzle, such as an axial direction and a cross-sectional area of an outlet port thereof, are simply adjusted while maintaining its conventional structure, it is unable to suppress the large spreading and deceleration of the molten steel flow and obtain the above required molten steel flow.
Specifically, as measures for solving the above problems, it has been tried to allow a molten steel flow discharged from an outlet port of an immersion nozzle to have fluidity required in the vicinity of the molten steel surface, even in the vicinity of the lateral end of the mold cavity, for example, by setting an axial direction of the outlet port of the immersion nozzle in an upward direction relative to a horizontal direction. In this immersion nozzle, the outlet port is formed in a part of a wall of a straight nozzle body thereof. Thus, even if the axial direction of the outlet port is variously adjusted under a constraint of a predetermined wall thickness of the straight nozzle body, it is unable to ensure sufficient fluidity in the lateral end of the wide mold cavity.
There has been known an immersion nozzle comprising a straight nozzle body, an outlet port portion protruding in a lateral direction slightly beyond a wall thickness of the straight nozzle body to serve as a means for controlling a molten steel flow, and a grid- or bar-shaped CaO-containing member primarily made of CaO and attached inside the outlet port portion, as disclosed, for example, in JU 63-085353A (Patent Publication 1). Although this immersion nozzle is designed to elongate the outlet port portion in the lateral direction so that a molten steel flow discharged from an outlet port in the outlet port portion can be directed in a desired direction, the molten steel flow is slowed due to the configuration of the outlet port bent at certain angle and the grid- or bar-shaped CaO-containing member disposed in the outlet port (it is rather intended to positively decelerate the molten steel flow). Thus, the immersion nozzle disclosed in Patent Publication 1 is incapable of allowing a molten steel flow required in the vicinity of the molten steel surface to be stably formed in a desirable range including the lateral end of the wide mold cavity.
JP 2004-344900A (Patent Publication 2) discloses an immersion nozzle comprising a canopy (or hood)-like member disposed above and/or below an outlet port thereof. Although this immersion nozzle provided with the canopy-like member can suppress formation of a downward flow, a molten steel flow is inevitably slowed and spread/decelerated particularly in a region having no canopy-like member. Thus, the immersion nozzle disclosed in Patent Publication 2 is incapable of allowing a molten steel flow required in the vicinity of the molten steel surface to be stably formed in a desirable range including the lateral end of the wide mold cavity.
All the above conventional approaches for controlling a molten steel flow based on the configuration of an outlet port of an immersion nozzle are not intended for the wide mold cavity, and a basic concept thereof is to positively slow or decelerate a molten steel flow in the mold cavity. That is, means for allowing a molten steel flow required in the vicinity of the molten steel surface to be stably formed in a desirable range including the lateral end of the wide mold cavity has not been yet disclosed.